ok, here is a scheme i think should be hard enough to break.
first i'll just post the crypted string, but you WILL need a hint, so i'll include one at the end.

now this is simplified a bit to make it a little easier to see what's going on(the numbers after each "-" should be made to look more randomish, which can be done quite easily)

and here are your hints:
1) the number after each dash indicates a position.
2) this evaluates to a string containing only letter(s) space(s) and period(s), but could easily be extended to include numbers and symbols

as a side note, this is a crypt that myself and some friends in HS thought up on our own, but i bet that the same basis has been used before(and probably quite a bit better).